1. Field of Invention
The invention relates to a magnetic random access memory (MRAM) and, in particular, to an MRAM with high write selectivity and low power consumption.
2. Related Art
Magnetic random access memory (MRAM) is nonvolatile memory. Using its magnetic resistance property to record information, it has the advantages of non-volatility, high densities, high access speeds, and anti-radiation.
The basic operation principles of the MRAM are the same as storing data on a hard disk drive (HDD). Each bit of data is determined by its magnetization orientation to be either 0 or 1. The stored data are permanent until being modified by an external magnetic field.
When data is read from the MRAM, an electrical current has to flow to a selected magnetic tunnel junction (MTJ) cell. The resistance value determines the numerical value of the data. When writing in data, a common method uses two wires (a bit line and a write word line) to induce a magnetic field at the selected cell, thereby changing the magnetization orientation of the magnetic material at the position and the data thereof.
The MTJ cell between the bit line and the write word line has a stack structure of multi-layer magnetic metal materials. The structure basically contains a soft magnetic layer, a nonmagnetic conductor or tunnel barrier, and a hard magnetic layer. Whether the magnetization orientations of the two layers of ferromagnetic materials are parallel or anti-parallel determines whether the stored datum is 1 or 0.
As memory devices are becoming smaller, the MRAM starts to encounter the electron migration problem because the write-in electrical current needed to change data approaches the current density limit that can be carried by a metal wire. To solve the problem, the U.S. Pat. No. 6,642,595 discloses a memory structure. As shown in FIG. 1, the MTJ cell formed from a magnetic tunnel junction cell 10 has a lower electrode 20 below it. An insulator 30 is below the lower electrode 20. The magnetic tunnel junction cell 10 is connected to the bit line 50 via a middle metal pillar 70. The write word line is composed of an upper layer write word line 40A and a lower layer write word line 40B. The induced total magnetic is further enhanced by the pillar write word lines (PWWL's) 60 on both sides of the magnetic tunnel junction cell 10. The PWWL's near the MTJ can produce a larger magnetic field focused on the MTJ. Therefore, the threshold current needed by the MRAM is reduced. Nonetheless, this structure has a problem. If the PWWL's point in the normal direction, they will produce a negative contribution to the magnetic field on the adjacent MTJ, enlarging the device size. Moreover, to avoid the interference of the write word line on MTJ's that are not selected, a better design for increasing the MRAM write selectivity is to let the magnetic field produced by the write word line pass through as few MTJ's as possible.